1. Field of the Invention
This invention pertains to a printing form and a process for preparing a printing form, and in particular, a process for preparing a polymeric gravure printing form in which one or more conventional metal layers are replaced by a curable layer of a specified multifunctional urethane.
2. Description of Related Art
Gravure printing is a method of printing in which the printing form prints from an image area, where the image area is depressed and consists of small recessed cups or wells, typically referred to as cells, to contain the ink or printing material, and the non-image area is the surface of the form. Gravure printing is particularly well suited for printing great numbers of copies of a desired image precisely and rapidly on a substrate and is widely used to print various articles, such as labels, flexible food packaging, wrapping paper, wall paper, boxes and cartons. A gravure cylinder, for example, is essentially made by electroplating a copper layer onto a base roller, and then engraving the image composed of the small recessed cells or wells digitally by a diamond stylus or laser etching machine. The cylinder with engraved cells is then mechanically ground and polished and overplated with a very thin layer of chrome to impart durability during the printing process. Consequently, metal gravure printing forms are expensive and require considerable time and material to produce. Furthermore, electroplating and etching processes used to produce metal gravure printing forms have pollution problems, since they generate considerable hazardous heavy metal wastes, and require costly waste disposal and regulatory permitting to operate.
Replacing the electroplated copper and chrome layers with a polymer-based composition has been explored, for example, by Aoyama et al. (U.S. Pat. No. 4,384,011), Bressler et al. (U.S. Pat. No. 5,694,852), Campbell and Belser (U.S. Patent Publication 2004/0221756), and Kellner and Sahl (UK Patent Application GB 2,071,574). However, a combination of several process and property requirements must be met for gravure printing forms having a polymer-based composition to succeed. For an economical process, a polymer-based coating needs to be applied to the cylinder easily (“coatability”) and cured reasonably rapidly (“curability”), allowing a high-quality surface layer to be produced to the strict tolerances required for gravure engraving and printing with a minimal requirement for grinding and polishing. The surface layer needs to have a level of hardness that produces well defined print cell structure when engraved, without significant chipping or breaking (“engravability”). The surface layer also needs to possess excellent resistance to the solvents used in gravure printing inks and cleaning solutions (“durability-solvent resistance”). Also, the surface layer needs to resist the mechanical wear (“durability-mechanical wear” and “scratch resistance”) encountered during the printing process. e.g., wear from the scraping of the doctor blade, wear from any abrasive particles that may be in the ink, and wear from the surface onto which the image is printed. Further, in order for gravure printing forms having a polymer-based composition to replace conventional metal-covered gravure printing forms, the polymer-based printing forms should be capable of relatively long print runs and provide a consistent printed image for a minimum of 100,000 impressions, and preferably at least 200,000 or more impressions. It is also desirable after printing with polymer-based printing forms for end users to be able to easily remove the polymeric layer so that the base support or cylinder can be reused.
As a consequence, there remains a need to identify specific polymer-based compositions with the desired combination of coatability, curability, and engravability that can be used to produce, in an economical and environmentally-friendly manner, a polymeric printing form precursor; and, for a polymeric printing form having the appropriate combination of solvent resistance, mechanical wear resistance, and print quality. It is also desirable for the polymeric layer to be easy to remove after use.